1. Field of the Invention
This invention relates to the formation of blood clots and specifically to the inhibition of blood clot formation and tissue damage following reperfusion.
2. Description of Related Art
The clotting of blood involves a cascade of enzymes, cofactors, and a group of cellular and plasma proteins known as coagulation factors. Initiation of this cascade occurs when the cellular receptor, known as tissue factor (TF), binds coagulation factor VII or its derivative, factor VIIa, to form a catalytically active complex. Factors X and IX. are activated by the TF-VIIa complex, thereby catalyzing thrombin generation and fibrin formation. TF is a membrane-bound glycoprotein that is not normally found soluble in the circulation or accessible to plasma proteins including factor VII/VIIa and the other coagulation factors.
TF is the principal procoagulant in the human brain. TF has been localized to the parenchyma of the adult human cerebral cortex, where it has a diffuse distribution (Drake, et al., Am.J.Pathol., 134:1087, 1989). The appearance of TF on stimulated endothelial cells and cells of the monocyte/macrophage lineage in vitro suggests a vascular association of the procoagulant, which is supported by the perivascular localization of TF antigen in non-neural tissues. Electron microscopy has demonstrated fibrin in microvessels associated with degranulated platelets/polymorphonuclear leukocytes, but not in capillaries following middle cerebral artery (MCA) occlusion/reperfusion (del Zoppo, et al., Stroke, 22:1276, 1991). The exposure of TF to plasma during vascular ischemia may contribute to intravascular coagulation defects.
TF, which is found predominantly in cerebral tissues and on perivascular cells, may be a contributor to the development of microvascular occlusions. In the brain, TF has a prominent perivascular distribution around non-capillary cerebral microvessels, especially in gray matter. Tissue factor is constitutively expressed on the surface of some extravascular cells in vitro including fibroblasts and certain epithelial cells that are separated from the circulating plasma proteins by basement membrane barriers. The presence of TF on these cells results in clot formation upon contact with blood and tissue damage thereby occurs.
Incomplete perfusion of the microvasculature following transient focal or global cerebral ischemia and reperfusion (I/R) constitutes the "no-reflow" phenomenon. Polymorphonuclear leukocytes and platelets, in addition to other endothelium and subendothelium-related mechanisms, have been implicated in the formation of these perfusion defects. Little is known concerning the role of fibrin formation, or other consequences of local thrombin generation, in causing microvascular obstruction following focal cerebral ischemia/reperfusion. In addition to cellular contributions to the microvascular perfusion defect following focal cerebral I/R, coagulation system activation may play a role. A role for the coagulation system in tissue damage has been suggested by studies showing the ability of the combination of heparin/ticlopidine to significantly reduce post-I/R microvascular occlusion formation and platelet deposition in a non-human primate model (del Zoppo, et al. Stroke 17(6):1254, 1986). Thus, methods which can inhibit reperfusion tissue damage would be of significant clinical value. The present invention provides such a method.